The present invention relates to interrogator/receiver apparatus for transmitting a first signal to, and receiving a second signal from a remote transponder. More particularly, the invention relates to interrogator/receiver apparatus for use with transponders which are capable of receiving an interrogating first signal, processing this signal and transmitting, in reply, a second signal that is derived from the first signal and contains encoded information.
Because the aforementioned encoded information normally includes an identification code which is unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels". The entire system, including the interrogator/receiver apparatus and one or more passive transponders, is therefore often referred to as a "passive interrogator label system" or "PILS".
Passive interrogator label systems of the type which the present invention relates are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,273,146 Horwitz, Jr. 3,706,094 Cole, et al. 3,755,803 Cole, et al. 3,981,011 Bell 4,058,217 Vaughan, et al. 4,059,831 Epstein 4,263,595 Vogel ______________________________________
Such a system is also disclosed in the commonly-owned patent applications referred to above.
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one passive transponder which receives this first signal, processes it and sends back a second radio frequency signal containing encoded information; and a receiver, normally located next to the interrogator, for receiving the second signal and decoding the transponder-encoded information.
In the aforementioned patent application Ser. No. 509,523, a passive interrogator label system is disclosed in which the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, a frequency band that is freely available in many parts of the world for short range transmissions.
A passive (i.e., nonpowered) transponder associated with this system receives the first (interrogating) signal as an input and produces a second (reply) signal as an output. Passive signal transforming means within the transponder, which converts the first signal to the second signal, includes:
(1) A multiplicity of "signal conditioning elements" coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal.
(2) A single "signal combining element" coupled to all of the signal conditioning elements for combining the intermediate signals to produce the second signal. This second signal is coupled out to the same or a separate antenna for transmission as a reply.
The signal conditioning elements and the signal combining element impart a known informational code to the second signal which identifies, and is associated with, the particular transponder.
The receiving and decoding apparatus associated with the system includes apparatus for receiving the second signal from the transponder and a mixer arranged to receive both the first signal and the second signal for performing four quadrant multiplication of these two signals. The mixer produces, as an output, a third signal containing the difference frequencies (or frequencies derived from the difference frequencies) of the first and second signals, respectively.
Finally, the system disclosed in the aforementioned U.S. patent application Ser. No. 509,523 includes a signal processor, responsive to the third signal produced by the mixer, for detecting the phases and amplitudes of the respective difference frequencies contained in the third signal, thereby to determine the informational code associated with the interrogated transponder.
This particular system has a number of advantages over passive interrogator label systems of the type disclosed in the issued U.S. patents referred to above. For example, this system exhibits substantially improved signal-to-noise performance over the prior known systems. Also, the output of the signal mixer--namely, the third signal which contains the difference frequencies of the first (interrogating) signal and the second (reply) signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are in the audio ranse. Furthermore, since the audio signal is not greatly attenuated or dispersed when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer.
In practice, a passive interrogator label system is frequently configured such that a plurality of transponders are interrogated from a number of locations. For example, if the transponders (labels) are carried on persons who are authorized entry into a building, the transmitting and receiving antennas are normally located near several doors to the building.
As another example, the labels may be placed on cattle which are monitored at a number of locations, such as a holding area, a feeding area and the like. The labels may also be placed on railroad cars to permit car identification at various locations throughout a switchyard or rail network.
In such configurations, it has been necessary to provide separate interrogator/receiver apparatus at each location from which the labels are to be monitored. As the number of locations increases, the equipment requirements and costs also increase dramatically.